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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

294                              ALASKAN GLACIEE STUDIES
Inlet, with a depth of 276 feet (Pig. 38), show differential glacial erosion in these narrow passages, for the main channel is deeper in each case. These small channels are analogous to hanging valleys, but are discordant at both ends.
In general, the soundings reveal Unakwik Inlet as a U-shaped glacial trough (Fig- 89) with about the same valley slopes below sea level as above. The fiord bottom, with depths of 34 to 1266 feet has a basined character, which is interpreted as, in part at least, due to glacial deposits.
The most striking of these irregularities is a barely submerged reef (Kg. 38) 10J miles south of Meares Glacier and just north of the mouth of Jonah Bay. At this point (M, Fig. 40), a reef rises 700 to 750 feet above the fiord bottom and extends con-tinously across the fiord, its presence being manifested at the surface by a pair of bars, each extending nearly half way across the inlet. Just west of the middle of the fiord, the channel has a maximum depth of about 15 feet at low tide, while at high tide the pair of projecting points is crossed by several channels. The presence of a shoal here was observed by Glenn in 1898 when his small steamer struck the reef in mid-channel.1
This shoal is thought to be a terminal moraine rather than a rock reef or a pair of sand spits built out from either shore, both because it rises out of deep water at a point where glacial erosion must have originally made the channel nearly if not quite as deep as it is to the north and south, and because the surface of the reef and the projecting
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FIG. 40.   NATUKAL SCALE PHOFILB or UNAKWIK INLET.
points on each side of the fiord, where not covered by sand, are made up of rocks of variable sizes and shapes, bearing glacial striations, and including masses far too large to have been transported here by any agency except a glacier. If the shoal is made up of morainic material from top to bottom, the presence of a moraine over 700 feet in height shows that the ice front must have stood at this point for a long time in order to accumulate so large a deposit, for the accumulation of glacial material in the water is thought to be very slow. At the surface the bar differs from the terminal moraine on the land in the absence of unassorted clay; but even though made in the main of till the absence of clay in the upper portion is easily explained by the tidal currents which Glenn estimated to run at the rate of 10 knots an hour. These might easily have washed away the clay and left only the large rocks, upon which sand spits have been built from either shore. On the basis of this interpretation the deposit may be referred to as a moraine bar.
Five miles south of this moraine, and just south of the mouth of Siwash Bay is a second rise in the bottom of the fiord (N, Fig. 40). It rises 156 to 228 feet above the fiord floor, although the water is 760 feet deep on its crest. It is impossible to state definitely whether this is a recessional moraine or whether it is a reef of more resistant rock which was left by glacial erosion as a result of glacial scooping just to the north. If the latter, the depression to the north (756 to 906 feet deep) is a rock basin produced
i Glenn, E. F., Reports of Explorations in the Territory of Alaska, No. XXV, War Dept., Adj.-Gen. Office^ 1899, p. 25.